1. Field of the Invention
The present invention relates to a method for driving a plasma display panel (PDP).
Television sets having a large screen of a PDP is becoming commonplace. As a resolution of a screen increases, a load of a power source circuit for a PDP in a display device becomes large. Therefore, countermeasures against the increasing load are requested.
2. Description of the Prior Art
An AC type PDP having three different fluorescent materials of different light emission colors is used for a color display. In the AC type PDP, display electrodes for generating display discharge that determines light emission quantity of cells are covered with a dielectric layer, and wall voltage that is generated by electrification of the dielectric layer is utilized for the display discharge. Among all cells within the screen, cells that are to generate display discharge are set to have higher wall voltage than other cells' wall voltage (usually zero volt). After that, a sustaining pulse train having amplitude lower than discharge start voltage is applied to every cell similarly. When a sum of the amplitude of the sustaining pulse and the wall voltage exceeds the discharge start voltage, display discharge is generated. At this time, ultraviolet rays are generated by a discharge gas and excite fluorescent materials in cells so as to emit light. The sustaining pulse is applied for approximately a few microseconds, and the light emission looks continuous.
The application of the sustaining pulse train by the driving device is performed for all cells at the same time after a line-sequential addressing step in which wall voltage in each cell of the screen corresponds to display data. A waveform of a usual sustaining pulse has a simple rectangular shape. Responding to the application of the sustaining pulse, display discharge is generated in all cells to be lighted substantially at the same time. Accordingly, concentrated discharge current flows temporarily from the power source circuit of the driving device to the plasma display panel. This concentration of the discharge current may cause a drop in amplitude of the sustaining pulse, i.e., a voltage drop, thereby the display distortion is generated. A power source circuit that can supply a current large enough to avoid the voltage drop is expensive, and it is not realistic to use such a power source circuit in the driving device.
A driving method that can relieve the concentration of the discharge current is disclosed in Japanese unexamined patent publication No. 2001-34227. In the method the waveform of the sustaining pulse is made a trapezoidal shape having a gentle voltage change at a leading edge. Since there is a little variation in the discharge start voltage among cells, some cells are relatively easy to start discharge but other cells are hard. Responding to the application of the sustaining pulse, display discharge begins at cells having low discharge start voltage at first. After that, display discharge begins at cells having high discharge start voltage. If the voltage change at leading edges of the sustaining pulse is gentle, a time point when display discharge begins in cells having high discharge start voltage is delayed compared with the case where the voltage change at leading edges of the sustaining pulse is sharp. In other words, since a start timing of the display discharge is dispersed in the entire screen, the concentration of the discharge current is relieved. Also, Japanese unexamined patent publication No. 2000-206928 discloses a method for dispersing the start timing of the discharge by making the waveform of the sustaining pulse be a step-like shape having a two-step voltage change at a leading edge of the sustaining pulse. Japanese unexamined patent publication No. 6-4039 discloses a circuit structure for relieving the concentration of current by shifting application timing for each of plural blocks constituting the screen.
There are problems in the conventional driving method. One of them is that power is consumed wastefully when the number of cells to be lighted is small and thus efficiency of light emission drops. Another problem is that ion bombardment received by the fluorescent material and the dielectric layer is larger when the number of cells to be lighted is small compared to the case when the number of cells to be lighted is large. As explained above, the dispersion of the start timing of the display discharge can reduce a peak value (a maximum instantaneous value) of the discharge current. However, the peak value of the discharge current is larger when the number of cells to be lighted is large compared to the case when the number of cells to be lighted is small. In addition, the voltage drop is more conspicuous if the current flows more. Therefore, it is necessary to determine the amplitude of the sustaining pulse in designing a drive condition in expectation of a voltage drop in the case where the number of cells to be lighted is large so that display discharge can be generated even if a voltage drop occurs. In this way, if the amplitude of the sustaining pulse is determined on the basis of the case where the number of cells to be lighted is large, voltage higher than necessary is applied to cells when the number of cells to be lighted is small. As a result, excessive display discharge occurs, efficiency of light emission drops, and cells may receive excessive ion bombardment.    Related Patent Publication 1:
Japanese unexamined patent publication No. 2001-34227    Related Patent Publication 2:
Japanese unexamined patent publication No. 2000-206928    Related Patent Publication 3:
Japanese unexamined patent publication No. 6-4039